Differential Diagnosis for 17-year-old Male with Knee Pain

Single Most Likely Diagnosis

• Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain behind the kneecap, especially with activities that put stress on the knee, such as climbing stairs, squatting, or jumping. The absence of pain at rest and the worsening of pain with activity over 4 months, despite home care, aligns with PFPS. The potential positive anterior drawer test could indicate some degree of ligamentous laxity or instability, which can contribute to PFPS.

Other Likely Diagnoses

• Osgood-Schlatter Disease: Although more common in younger adolescents, this condition can persist into late teens. It involves pain, swelling, and tenderness below the kneecap due to inflammation of the patellar tendon. Activity-related pain and a positive anterior drawer test could be seen in this condition, especially if there's associated knee instability.
• Patellar Tendinopathy: This condition involves inflammation of the patellar tendon, leading to pain below the kneecap. It's common in athletes who participate in sports involving jumping. The worsening pain with activity could suggest patellar tendinopathy, especially if the pain is localized to the inferior pole of the patella.

Do Not Miss Diagnoses

• Osteosarcoma or Other Bone Tumors: Although rare, bone tumors can present with progressive pain that worsens over time. The absence of pain at rest does not rule out this diagnosis, as some bone tumors can cause pain only with weight-bearing or activity. A positive anterior drawer test might not be directly related but could indicate a mass effect or bone weakening.
• Septic Arthritis or Osteomyelitis: Infection within the joint (septic arthritis) or bone (osteomyelitis) can cause severe pain, especially with movement. While the presentation might not fully align with these conditions (e.g., systemic symptoms like fever are often present), they are critical to rule out due to their potential for severe consequences if not promptly treated.

Rare Diagnoses

• Sinding-Larsen-Johansson Syndrome: This is a rare condition involving inflammation of the patellar tendon at its origin on the inferior pole of the patella. It's more common in younger athletes and can cause activity-related knee pain.
• Discoid Meniscus: An abnormal shape of the meniscus can lead to knee pain, especially with twisting or bending movements. While it might not directly cause pain behind the kneecap, it could contribute to overall knee instability or pain, potentially worsening with activity.